XPost: sci.electronics.basics, sci.electronics.design, sci.electronics.misc   
   From: andrew@cucumber.demon.co.uk   
      
   In article <540AE8AB.9020801@electrooptical.net>,   
   	Phil Hobbs  writes:   
   > On 9/6/2014 2:43 AM, piglet wrote:   
   >> On 06/09/2014 05:51, Fester Bestertester wrote:   
   >>> AC voltage ballasts are superior to DC ballast.   
   >>   
   >> I would think the ebay sellers desciption is roughly   
   >> right. In a DC arc the electrode material will "burn   
   >> off" and migrate only in one direction. In AC arc the   
   >> wear reverses each cycle and cancels out. This was   
   >> known from carbon arc lights back 150years ago.   
   >>   
   >> I am surprised that any headlamp Xenon arcs are DC   
   >> and if they were DC arc then lifetimes would be   
   >> very short.   
   >>   
   >> piglet   
   >>   
   >   
   > Normal arc lamps are asymmetrical--they run from DC, and fail quickly if   
   > you put them in backwards. Dunno about HIDs.   
      
   Yes, depends on the design of the lamp, and where exactly the   
   useful light source is inside it.   
      
   Light source can be the positive column (as with long tubes),   
   cathode fall region (neon indicators), or an electrode surface   
   (carbon arc).   
      
   If the lamp is designed for DC, the electrodes could well be   
   asymmetrical, designed to handle different operating temperatures,   
   different thermionic emission properties, different wear rates,   
   etc. An asymmetric DC lamp operated on AC would have shorter life.   
      
   Also, any lamp operated on AC may have a larger light source,   
   which if the optics were not designed for this may cause light   
   spill out of the designed beam pattern and less intensity inside   
   the designed beam pattern. It *may* be that some headlamps use   
   DC to get a smaller more intense light source, which can be more   
   accurately focused by beam optics into the required pattern, to   
   achieve higher intensity in-beam and less spill outside the   
   intended beam.   
      
   The main point is the lamp should be operated on AC or DC, which-   
   ever it was designed for (including the design of the lamp optics,   
   headlamp reflectors and lenses).   
      
   Electrode wear does not all end up on the other electrode. In   
   many lamps none of it does (it ends up spluttered over the inside   
   of the arc tube), but in some cases this can be avoided (e.g. the   
   hollow cylindical electrodes of cold cathode signage tubes, where   
   most of it is retained within the electrode itself).   
      
   --   
   Andrew Gabriel   
   [email address is not usable -- followup in the newsgroup]   
      
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